Artificial food casings, including plastic casings and cellulose casings, have been used for many years as containers in which food products are manufactured and stored. In commercial applications, the food casings are generally loaded either by hand or automatically into a food stuffing machine in order to stuff food products, such as sausage, meats, vegetables, or other food products, into the casings. In order to increase the length of casing that can be utilized at one time, casings have for years been compressed through ruffling or other compression techniques, into short, compact lengths, called shirred sticks or shirred strands.
Casings can be natural or artificially manufactured. Artificial casings generally fall within one of four categories: skinless cellulose casings (small calibers, pure regenerated cellulose), collagen casings (edible, non-edible, animal derivative), plastic casings (typically Nylon and PE), and fibrous casings (viscose coating on hemp paper). These casings can be used in reels, in cut pieces, or shirred (compacted) form.
Current technology for shirring skinless cellulose casings is generally disclosed by U.S. Pat. No. 3,454,982 issued to Arnold. Arnold discloses a shirring method and system that includes a plurality of shirring rolls (also called wheels or gears) that engage a casing that is inflated about a mandrel and fed through the shirring rolls. The shirring rolls have angled teeth so that as the rolls rotate, the teeth of successive wheels engage the casing to form a helical pleat, applying a vector force at a direction not parallel to the shirring mandrel. Typical skinless casings are shirred with 18%-23% moisture by weight, and do not require additional soaking prior to stuffing with food products.
Collagen casings, which are often edible and therefore are more delicate than skinless cellulose casings, are less adaptable to automatic stuffing processes. Accordingly, fewer innovations in shirring and compaction of the casings have been pursued. Typical methods for shirring collagen casings are disclosed by U.S. Pat. Nos. 3,209,398 and 4,550,472.
Polymeric plastic casings, which are gaining popularity, may be used without any soaking prior to stuffing, or may require soaking after shirring but prior to stuffing. Several shirring techniques exist for shirring plastic casings, the most notable disclosed by U.S. Pat. Nos. 3,988,804, 3,454,982, and 4,377,885.
Fibrous casings are generally manufactured by coating a hemp paper with viscose, which is then further regenerated into cellulose, and are sold as one of two types of casings. A large percentage of fibrous casing is typically pre-moisturized to 16%-18% before shirring. After shirring, the casing must be soaked to a moisture content of about 35% to 45% by weight to allow full wetting of the cellulose fibers prior to stuffing, especially important to provide elasticity to the hemp. Although both skinless and fibrous casings are cellulosic, fibrous casings are very thick, typically around 100 microns, which is about four times the thickness of skinless casings. This high thickness and stiffness of fibrous casing require adequate moisturizing to provide elasticity for functioning. Shirring techniques have generally been limited for fibrous casings of this type due to the effects of excessive compaction of casing within the shirred stick, which impedes soakability. Excessive compaction decreases the space within the pleats, thus preventing water in the soak tank from moving into the volume of the shirred stick in sufficient amounts in order to be absorbed by the cellulose. Typical shirring techniques for fibrous casings that require additional soaking after shirring are disclosed in U.S. Pat. No. 4,377,885.
The second type of fibrous casings are premoisturized prior to shirring and require no additional soaking before stuffing. Due to the fact that no additional soaking is required prior to stuffing, higher compression of the shirred stick prior to doffing from the shirring machine is a typical method of increasing the length of casing that can be loaded into a stuffing machine, because there is no further requirement for soakability, and thus no requirement for spacing between the pleats after shirring. Still, U.S. Pat. No. 4,377,885 is the most typical casing used for shirring pre-soaked fibrous casings although U.S. Pat. No. 3,988,804 is also used in selected applications.
A variety of shirring technologies therefore exist. Generally, a flattened casing is inflated with compressed air, allowing it to be fed into a shirring machine from a reel through feedrolls. As the casing moves through the feedrolls, the casing is inflated around the mandrel and engaged by mechanisms including screws, belts or teeth which are part of the shirring rolls, these shirring rolls being mounted inside a fixture called a shirring head, that pull the casing into pleats. A variety of shirring heads exist. Newer versions of the shirring heads include a plurality of shirring rolls, with each shirring roll having a plurality of teeth. Shirring heads typically attempt to achieve a continuous helical pleat implanted onto the casing by the rotation of the shirring teeth on the shirring rolls. Certain shirring heads also simultaneously rotate the shirring head around the mandrel, such as U.S. Pat. No. 4,377,885 to assist in forming this helix.
Various improvements have been made over the years with respect to shirring techniques. Though effective in increasing the compaction of casing within a shirred stick, the resultant diameter of a shirred fibrous stick is typically between 95% to 112% of the inflated diameter of the casing. The volume within the shirred stick available for packing casing depends upon shirred stick outer diameter, shirred stick inner diameter and shirred stick length, as shown in, U.S. Pat. Nos. 4,590,749 and 5,358,765. These references illustrate that for a given casing, a calculation can be made using shirred stick dimensions to estimate the density of casing compacted into the shirred stick.
In the case of fibrous casing requiring soaking prior to stuffing with meat, different types of casings require different densities (pack efficiency) within the shirred stick, to allow water to penetrate into the stick permitting full soaking. The shirred stick length and inner diameter are generally set in value according to the type of stuffing equipment used, therefore leaving the shirred stick outer diameter as the only free variable around which to increase the available volume into which casing can be packed. Various shirring techniques are able to pull the casing in a manner to form various shirred stick outside diameters. For a given type of casing shirred into a stick with a given shirred stick length and a given shirred stick inner diameter, a greater outer diameter of a shirred stick allows more available volume into which casing can be compacted, allowing either more free air volume for a given casing length (easier soakability), or conversely allowing more casing to be compacted into the stick (with equal soakability or for improved length on non-soak articles).
A significant limitation of the current technology is that casings shirred today using conventional methods do not permit long lengths of casing to be compacted within the shirred stick and still achieve adequate soaking with a desired shirred stick length and a desired shirred stick inner diameter. The excessive density of casing packed into the stick often prevents sufficient wetting of the casing during soaking.
As shirring mandrels increase in size relative to the casing diameter, to provide larger shirred stick inner diameters allowing stuffing machines to increase throughout by using larger diameter stuffing tubes, the pleats within the shirred stick geometrically become shorter and harder to pull, with less surface area available within the pleat for teeth on shirring rolls to grab the casing. With current techniques that use larger mandrel diameters relative to the casing inflated diameter, the shirring rolls are required to rotate with extremely high tooth velocities relative to the velocity of the casing in the shirring machine feedrolls in order to grab and pull the shorter pleat. Force applied is proportional to velocity squared, so high differential velocity between the shirring teeth and the feedrolls greatly improve forces to adequately grab the casing. The difference in velocity of the shirring tooth verses the velocity of the casing at the feedrolls is generally referred to as “overshirr.”
The result of the high overshirr of the shirring rolls required to pull the casing is that a high number of relatively short pleats are formed, including forming a large number of non-uniform or “nuisance” pleats that create stress on the casing due to irregular folding, impeding water absorption by blocking passageways for water penetration into the stick by capillary action, and retarding efficiently compacting long lengths of casing into the shirred stick without damage, since each pleat or fold adds corresponding nuisance pleats. This is further impeded because as the casing absorbs water, the cellulose structure swells up, and the fibrous casing thickness often increases by 50% to 100% of its original thickness, further restricting passage of water into the interior of the shirred stick. Once the passageways close due to casing swelling, absorption of water stops, limiting the casing at some final moisture level below desired full wet-out which is required to achieve sufficient wettability. With fibrous casings shirred by prior art, the length of casing that could be shirred into the stick was limited below some desired value, to reduce compaction to allow improved soakability, allowing more free air space around the pleats for water penetration into the shirred stick.
When the shirred stick is deshirred (i.e. unpleated by extending the shirred stick to its full length without stretching the casing material), the helical pattern of the pleat ridge can be seen. The distance between major pleats along the longitudinal axis may be called the pleat pitch. Additionally, the existing technology, as described above, typically produces a pleat length or “pleat pitch” in fibrous casing that is only 50% to 80% of the dry flat width of the casing, using conventional shirring mandrels with ratio of shirring mandrel diameter to inflated casing diameter of 60% to 85%. As the shirring mandrel increases in diameter, the pleat pitch generally shortens due to geometry of the stick. Commercial shirring mandrel diameters range from 60% to 85% of the deshirred casing's diameter, that diameter being measured with low pressure inflation or determined mathematically from the lay-flat condition.
In addition to the problems noted above with respect to compaction, excessive abrasion and damage resulting in pinholing occurs when the shirring rolls and teeth have a much higher velocity than the velocity of the casing fed from the feedroll (i.e.: high overshirr). In the case of high overshirrs, abrasions on the surface of the casing are a significant problem aggravated by the excessive differential velocity of shirring tooth verses incoming casing. Using current technology, if overshirr is excessive with a given number of teeth, creating excessively high shirring tooth velocities relative to the feedroll velocity, the only method of operating with reduced overshirrs and still obtain a desired pleat pitch is to add more shirring teeth to the shirring roll, which will help to shorten the pleat length for a given velocity to conform to the ideal geometric requirement, but has limited pulling power since shorter pleats which are harder to pull typically require higher overshirrs just to grab the casing. Thus, although adding teeth results in reducing overshirr requirements, serving to shorten the pleat pitch as geometrically required when shirring mandrel diameter increases relative to the casing inflated diameter, this action also greatly reduces the pulling power of the shirring head.
An alternate to conventional technology disclosed in U.S. patent application Ser. No. 10/398,244 to Kollross uses a vacuum assist to create a pleat pitch and stick outer diameter greater than conventional art, but has only demonstrated this technology with very small ratios of shirring mandrel diameters to inflated casing diameter, such as 50% to 60%. With respect to Kollross's vacuum assist shirring, the very slow speeds at which the shirring machine overall operates, creating a very low productivity coupled with the enormous power requirements for vacuum shirring result in a shirring method that is not commercially viable. Accordingly, conventional technology remains the most secure method to shirr casing for all varieties of casings and mandrel combinations.